XXL-HSC: An updated catalogue of high-redshift (z3.5) X-ray AGN in the XMM-XXL northern field: Constraints on the bright end of the soft log N -log S
Journal article, 2022

X-rays offer a reliable method to identify active galactic nuclei (AGNs). However, in the high-redshift Universe, X-ray AGNs are poorly sampled due to their relatively low space density and the small areas covered by X-ray surveys. In addition to wide-area X-ray surveys, it is important to have deep optical data in order to locate the optical counterparts and determine their redshifts. In this work, we built a high-redshift (z3.5) X-ray-selected AGN sample in the XMM-XXL northern field using the most updated [0.5-2 keV] catalogue along with a plethora of new spectroscopic and multi-wavelength catalogues, including the deep optical Subaru Hyper Suprime-Cam (HSC) data, reaching magnitude limits i 26 mag. We selected all the spectroscopically confirmed AGN and complement this sample with high-redshift candidates that are HSC g- and r-band dropouts. To confirm the dropouts, we derived their photometric redshifts using spectral energy distribution techniques. We obtained a sample of 54 high-z sources (28 with spec-z), the largest in this field so far (almost three times larger than in previous studies), and we estimated the possible contamination and completeness. We calculated the number counts (log N-log S) in different redshift bins and compared our results with previous studies and models. We provide the strongest high-redshift AGN constraints yet at bright fluxes (f0.52 keV>10 15 ergs 1 cm2). The samples of z 3.5, z4, and z5 are in agreement with an exponential decline model similar to that witnessed at optical wavelengths. Our work emphasises the importance of using wide-area X-ray surveys with deep optical data to uncover high-redshift AGNs.

Methods: data analysis

Methods: observational

Galaxies: active

Methods: statistical

X-rays: galaxies

Early Universe


E. Pouliasis

National Observatory of Athens

I. Georgantopoulos

National Observatory of Athens

A. Ruiz

National Observatory of Athens

R. Gilli

INAF OAS Bologna

E. Koulouridis

National Observatory of Athens

M. Akiyama

Tohoku University

Y. Ueda

Kyoto University

L. Chiappetti

Istituto nazionale di astrofisica (INAF)

C. Garrel

University Paris-Saclay

Cathy Horellou

Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics

T. Nagao

Ehime University

S. Paltani

University of Geneva

M. Pierre

University Paris-Saclay

Y. Toba

Academia Sinica

Kyoto University

Ehime University

C. Vignali

INAF OAS Bologna

University of Bologna

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 658 A175

Subject Categories

Astronomy, Astrophysics and Cosmology

Atom and Molecular Physics and Optics




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